1. Field of the Invention
The present invention relates to methods and apparatus for measuring the amplitude and phase of an RF test signal in general and in particular to a method and apparatus for use in a vector network analyzer comprising a time varied profile pulse and narrow bandwidth filter for measuring the magnitude and phase of the spectral lines in an RF stimulus pulse.
2. Description of the Prior Art
A vector network analyzer is used the electrical characteristics, i.e. the forward and reverse S-parameters, of a device, i.e. electrical network, having an input port and an output port. The S-parameters are obtained by selectively applying a reference signal RA and RB to the input and output ports and measuring the signals, called test signals, reflected from and transmitted through the device in response thereto. The reflected and transmitted test signals from the input port are called TA and those from the output port are called TB. The following equations define the S-parameters: ##EQU1## The reference signal may be a continuous wave (CW) signal or a phase-coherent RF pulse, also called a stimulus pulse.
In a prior known network analyzer which uses a CW reference signal, the CW test signal which is reflected from or transmitted through the device-under-test (DUT) is detected using a synchronous detector. The synchronous detector provides a pair of dc output signals, the amplitudes of which correspond to the real R.sub.e and imaginary I.sub.m components of the test signal represented by the equations: EQU R.sub.e= A cos.phi. (5) EQU I.sub.m= A sin.phi. (6)
where
A=the amplitude of the test signal; and PA1 .phi.=the phase of the test signal relative to the reference signal
From equations (5) and (6) the average amplitude A and phase .phi. can be obtained from the following: ##EQU2##
Detecting the amplitude and phase of a CW test signal using a synchronous detector simply requires a system having a sufficient bandwidth centered at the frequency of the CW test signal. This is because, as shown by a Fourier analysis, a CW test signal comprises in the frequency domain a single spectral line at the frequency of the CW test signal. Detecting the amplitude and phase of a phase-coherent pulsed-RF test signal, however, is more complex.
A train of phase-coherent RF test pulses comprises in the frequency domain, as shown by a Fourier analysis, a plurality of uniformly spaced spectral lines and nodes. The amplitude of each line corresponds to the contribution of that line to the amplitude and phase of the entire pulse. The spacing between the lines is a function of the cycle time T, i.e. the pulse repetition frequency .sup.1 /T and the spacing of the first nodes from the center frequency and each adjacent node is a function of the period of the pulse t.sub.p, i.e. .sup.1 /t.sub.p.
If the bandwidth of a system is broad enough to detect all of the spectral lines of an RF test pulse simultaneously, the resulting output comprises an instantaneous value of the magnitude and phase of the test pulse, but the signal-to-noise ratio of the detected signal is relatively low as a result thereof. On the other hand, if the bandwidth of the system is reduced so as to encompass less than all the spectral lines, the signal-to-noise ratio is improved, but a less accurate measure of the amplitude and phase of the test signal is obtained.
In a prior known pulsed-RF network analyzer, the test pulses reflected from or transmitted through a DUT are down-converted to a 20 MHz intermediate frequency (IF), passed through a broadband amplifier having a bandwidth of 3.0 MHz and detected in a synchronous detector. The outputs from the detector comprising the real R.sub.e and imaginary I.sub.m components of the detected test signal as described above are then sampled in a sample-and-hold circuit and converted to a digital signal in an analog-to-digital converter.
In a specific embodiment of the above-described pulsed-RF analyzer, the pulse period t.sub.p is 1 microsecond (.mu.sec) so that in the frequency domain the spectral lines have a uniform spacing of .sup.1 /T and the spectral nodes in the frequency spectrum are uniformly spaced from the center frequency by .sup.1 /t.sub.p, i.e. 1 MHz. With a system bandwidth of 3.0 MHz, the number of spectral lines detected is restricted to the principal lobe centered at 20 MHz and one half of each of the adjacent side lobes.
In operation, a sampling pulse of 100 nanoseconds (100 nsec) is applied to the sample-and-hold circuits and moved across the profile of the detected pulse to obtain a measure of the amplitude and phase of the detected pulse within each period of the sampling pulse.
While theoretically capable of measuring the amplitude and phase of a single pulse, the above-described prior known pulsed-RF analyzer has several disadvantages.
In order to obtain an accurate measurement of the amplitude and phase of a detected signal, it is necessary to detect all of the spectral lines in the detected pulse. However, as described above, the prior known system detects only those lines within the principal lobe and one-half of each adjacent side lobe, i.e. within the 3.0 MHz spectral bandwidth. Consequently, a number of spectral lines are not detected.
A system having a bandwidth of 3.0 MHz as described above necessarily restricts the minimum RF pulse duration to about 1 .mu.sec. This is because narrower RF pulses would increase the spacing of the spectral lobes detected and hence reduce the accuracy of the desired amplitude and phase information. This limitation is significant because modern RF signal processing systems, such as radars, routinely use stimulus pulses having a period of much less than 1 .mu.sec.
A further disadvantage of the above-described pulsed-RF analyzer is that currently available sample-and-hold circuits require a sampling pulse having a period of at least 100 nsec. With a period of that length, the sampling of an RF pulse having a period of 1 .mu.sec. is limited to 10 samples. Furthermore, a sampling period as long as 100 nsec. does not allow for an accurate sampling of leading and trailing edges of the test pulse.
Still another disadvantage of the above-described prior known pulsed-RF analyzer is that, having a broad system bandwidth, its dynamic range is restricted, tests have shown, to about 60 db.